Broad spectral response pickup tube

ABSTRACT

A broad spectral response pickup tube incorporating a photoemissive cathode responsive to a first range of wavelengths of radiation with proximity focusing providing for the electron emission from the photocathode onto an electron responsive target. The input window and the photoemissive cathode being transmissive to a second range of wavelengths of radiation focused onto said electron responsive target in which said electron responsive target is also responsive to the second range of radiation wavelengths.

United States Patent [191 Beyer et al.

[ 51 July 24,1973

[ BROAD SPECTRAL RESPONSE PICKUP TUBE [75] Inventors: R011 R. Beyer, Horseheads; Peter R. Collings, Ithaca; Alfred B. Laponsky, Horseheads, all of N.Y.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Aug. 4, 1971 [21] Appl. No.: 168,912

3,457,451 7/1969 Manley 315/10 X 3,491,233 1/1970 Manley 315/10 3,575,628 4/1971 Word 250/213 VT X 3,619,496 11/1971 Lichtenstein 250/213 VT X 3,634,692 1/1972 Padonani et al 250/213 VT X Primary Examiner-Carl D. Quarforth Assistant Examiner-P. A. Nelson Attorney-F. H. Henson, C. F Renz et al.

[57] ABSTRACT A broad spectral response pickup tube incorporating a photoemissive cathode responsive to a first range of wavelengths of radiation with proximity focusing providing for the electron emission from the photocathode onto an electron responsive target. The input window and the photoemissive cathode being transmissive to a second range of wavelengths of radiation focused onto said electron responsive target in which said electron responsive target is also responsive to the second range of radiation wavelengths.

5 Claims, 3 Drawing Figures PATENIEU JUL 2 4 I975 Own;

hm vm mm a m 985 O BOJ N mmlw at %n TMW OIL Wm (M BROAD SPECTRAL RESPONSE PICKUP TUBE BACKGROUND OF THE INVENTION There are several pickup tubes presently available for low light level imaging of scenes. These pickup tubes generally provide high quantum efficiency, spectral response to wavelengths of radiation between 0.4 micrometer and 0.85 micrometer and a high electron gain. Some examples of these pickup tubes are the secondary electron conduction (SEC) tube, and EBS tube with an intensifier section and an electron bombarded silicon diode (EBS) type target. The EBS pickup tube offers particularly high electron gain within the target.

Limitations in the use of these pickup tubes are imposed by the spectral response of the photocathode and faceplate combination. The low wavelength regions response of the tube can readily be extended by employment of an ultraviolet transmissive input window. In the wavelength region greater than about 0.85 micrometers, the photocathode quantum efficiencies become extremely low. Thus the above pickup tubes employed in low light level applications do not operate effectively over the entire spectral region from the ultraviolet to the infrared.

There are certain devices and particularly solid state type sensors which offer high quantum efficiencies, greater than photoemissive sensors, at longer wavelengths. One example is the silicon diode array target vidicon wherein the light is directly focused onto the target rather than converting the light into an electron image as in the E88 type device. This light sensitive silicon diode array target vidicon is capable of operating up to a wavelength of at least 1.1 micrometers and at 1.06 micrometers can have quantum efficiencies as high as 8 to 10 percent. This device in itself offers the desired broad spectral response but has at most unity gain. Therefore, this type of tube is not sufficiently sensitive for most low light level television applications.

SUMMARY OF THE INVENTION This invention is directed to a pickup tube which incorporates; into a single device, a photoemissive sensor and a photoconductive sensor. The employment of the two sensors is made possible by means of proximity focusing. The pickup tube is comprised of a photocathode provided on the input window with a target closely spaced thereto and a reading electron gun provided on the opposite side of the target with respect to the photocathode. The target is responsive to both electrons, and input radiation directed onto the pickup tube. The pickup tube can operate into a photoemissive mode, a photoconductive mode or alternately in a mode in which both methods of sensing the incident radition are employed simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention reference may be had to the preferred embodiment, exemplary of i the invention, shown in the accompanying drawings, in

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. I, there is illustrated a pickup tube comprised of an envelope 10 including a tubular body portion 12 having a button stem 14 provided at one end thereof for closing off that end of the tubular portion 12. The button stem 14 also includes a plurality of lead-ins 16 for applying potential to the electrodes within the envelope 10. The other end of the tubular member 12 is closed off by an enlarged cylindrical envelope portion 18. A faceplate 20 is provided within this enlarged en velope portion 18. The faceplate 20 is of a suitable material transmissive to radiations from the ultraviolet range through the infrared range and may be of a suitable material such as lithium fluoride, magnesium fluoride or quartz. A photocathode 22 is provided on the inner surface of the faceplate 20. The photocathode 22 may consist of a thin layer of about 0.01 micrometers of a suitable multi-alkali photocathode such as 5-20. The photocathode 22 will absorb radiations directed thereon from a scene 23 and focused thereon by a suit able lens system 25. The photocathode 22 will particularly absorb radiations in the ultraviolet and visible range and is substantially transparent to the infrared range. A target member 24 is provided adjacent to and parallel to the photocathode 22. The spacing between the photocathode 22 and the target 24 may be about I to 5 millimeters.

The target structure 24 may be of any suitable type responsive to both electrons and light radiation. The target 24 shown herein is a diode array type structure and is shown in detail in FIGS. 2 and 3. The target 24 may be a diode device which may be in the form large area heterojunction or a diode array. The target 24 is comprised of a wafer or substrate 26 of a suitable semiconductive material such as silicon, germanium or indium arsenide. The specific device shown utilizes an N-type silicon material having a (resistivity) of about 10 ohm-centimeter. The thickness of the active portion of the wafer may be about 10 to 25 micrometers and the target may have a diameter of about 25 mm. The thickness must be adequate to absorb infrared radiation. A mosaic of P-type regions 30 is provided on the side of the wafer 26 facing an electron gun 40 and forms a P-N junction 31 between the mosaic regions 30 and the wafer 26 to provide a plurality of diodes 27. A layer 32 of silicon dioxide is provided over the wafer 26 on the side facing the electron gun 40 and opened areas are provided therein through which the P-type elements 30 are formed. Electrically conductive contacts 34 are provided on the P regions 30. The contacts 34 may be of a suitable material such as gold, aluminum or chromium. The contacts 34 are reflective for infrared radiation. An N|+ layer 28 is provided on the surface of the wafer 26 facing the photocathode 22. i

The target 24 may be fabricated from an 0.008 inch, N-type silicon slice. The silicon slice 26 is oxidized by heating at 1,000C. for a few hours to provide the layer 32. By suitable photolithographic techniques the openings are provided in the layer 32. The wafer is then placed in a boron diffusion furnace and the boron diffuses through the openings in layer 32 to form the regions 30 and the P-N junctions 31. The wafer is then thinned to the desired thickness and then a P 0 gettering process is used to remove deep lying impurities and also to form an N+ layer 2 8. The resistive layer 34 is then deposited on the diode surface. A general description is found in Photoelectronic Imaging Devices Volume 2, published by Plenum Press 1971.

The diodes 25 may be about 25 micron on centers with a spacing between the P-regions 30 of about microns and the openings in the layer 32 may be about microns.

The electron gun 40 is provided at the opposite end of the envelope with respect to the target 24 and generates a pencil size electron beam for scanning a raster over the target 24. The electron gun 40 is comprised of a cathode 42 which may be connected to ground potential. The electron gun 40 may also comprise control grid 44 and a focusing electrode 46. A grid member 48 maybe provided adjacent to the target member 24 and connected to a slightly higher potential than the electrode 46. The potential of the electrode 48 may be about 400 volts. The electron gun 40 may be focused by either electrostatic or electromagnetic means. An electromagnetic focusing coil 50 is provided about the outer portion of the tubular member 12. The deflective means may also be electrostatic or electromagnetic and an electromagnetic coil 52 is illustrated for deflection of the electron beam to scan a raster over the target 24.

The photocathode 22 may be connected to a switching member 56 to permit connection of the photocathode 22 to a high negative potential of about 10 kilovolts from the source 58 or to a low voltage source 60 which may be substantially ground potential. The target member 24 is provided with a peripheral electrical conductive member contacting the peripheral portion of the NH- region 28 of the target 24 and is connected by a suitable lead-in 62 to the external portion of the envelope 10. The lead-in 62 is connected through a resistor 64 to a suitable battery 66. The negative terminal of the battery 66 is connected to ground and the battery 66 may be of a potential of 10 to volts. The video signal is derived from the target 24 across the resistor 64.

In the operation of the device, the pickup tube may either operate in a photoconductive mode or simultaneous photoemissive and photoconductive modes. In the photoconductive mode, the photocathode 22 would be connected to the low potential source 60. The radiation from the scene 23 is focused by the lens onto the target 24. The radiation'in the infrared region is transmitted through the faceplate 20 and the photocathode 22 onto the target 24. The contacts 34 also reflect infrared radiation and increase response of the target 34 to the infrared radiation. The electron beam from the electron gun initially established and periodically reestablishes a reverse bias on the P-N junctions 31. The radiation is absorbed by the silicon diode array target 24 and produces a corresponding pattern of electron-holes in the wafer 26. The holes diffuse to the junction 31 and partially discharge the reverse bias. The electron beam will recharge on the next scan and will produce an output signal pulse to the video output. The operation is such that the electron gun 40 tends to charge the elements 30 of the junctions 31 of the target 24 to ground potential while the backplate 28 is at a positive potential of 10 to 20 volts. The P-N junctions 31 have a reverse bias and therefore when the light strikes the wafer 26 the element 30 will charge in the positive direction.

In the simultaneously 7 photoemissivephotoconductive mode, the photocathode 22 will be connected to the high potential source 58 by the switch 56. Radiation from the scene 23 is directed through the lens 25 onto the photocathode 22. A portion of the visible radiation is absorbed by the photoemissive cathode 22 and electrons will be generated which are then accelerated toward the target 24. The remainder of the visible radiation and most of the infrared radiation is transmitted by the photocathode 22 and falls directly on the target 24 exciting electron-hole pairs as previously described. Both the photoelectrons and photons incident on the target 24 produce electron-hole pairs in the wafer 26. The operation of the target 24 is similar with respect to photon or electron input. Since both sensors, the photocathode 22 and the target 24, are separated by small but finite proximity focusing spacing, an objective lens 25 with corresponding depth of focus is required if high resolution for both types of radiation is desired.

The pickup tube described provides a broad spectral response providing a high sensitivity in the visible or ultraviolet region of the spectrum and also in the infrared radiation spectrum. The sensitivity to visible and ultraviolet radiation may be adjusted over a wide range independent of the sensitivity to the infrared. The tube is also adapted to provide that each of the two basic sensitivity regions can in some degree be independently optimized in their primary spectral region. The choice of the photocathode and the faceplate material determines the sensitivity and cut-off characteristics of the visible light or ultraviolet region. The infrared sensitivity can be adjusted by incorporating infrared antireflective layers at the target input surface optimizing the target thickness and proper target design.

We claim as our invention:

1. A pickup tube comprising an evacuated envelope, a faceplate provided for transmission of input radiation, a photocathode provided on the inner surface of said faceplate responsive to a first portion of said input radiation to generate photoelectrons, a target member positioned adjacent to said photocathode and means for accelerating the photoelectrons emitted by said photocathode into incidence with said target member to generate a charge image on the opposite side of said target with respect to said photocathode corresponding to the electron excitation, said target positioned in proximity to said photocathode to provide focusing of the photoelectrons emitted by said photocathode between said photocathode and said target, said faceplate and said photocathode transmissive to a second portion of said input radiation including the infrared region directed onto said pickup tube and focused onto said target member, said target member exhibiting a response to said second portion of said input radiation directed thereon by generating a charge image on the opposite surface of said target with respect to said photocathode corresponding to the radiation within said second portion, and means for generating and directing an electron beam over the opposite side of said target with respect to said photocathode to derive an electrical signal representative of the resultant charge image.

2. The device set forth in claim 1 in which said target member comprises a diode.

3. The device set forth in claim 1 in which said target comprises a semiconductive wafer including a plurality of P-N junctions localized near the surface of said target and remote from said photocathode, means for reverse biasing said P-N junctions comprising said means for generating said electron beam and means for derivcomprises a semiconductive wafer including a plurality of regions of semiconductive material localized near the surface of said target and forming P-N junctions with said wafer, said regions on the surface of said wafer remote from said photocathode, electrical conductive contacts provided on said regions and reflective to radiations in the infrared region. 

1. A pickup tube comprising an evacuated envelope, a faceplate provided for transmission of input radiation, a photocathode provided on the inner surface of said faceplate responsive to a first portion of said input radiation to generate photoelectrons, a target member positioned adjacent to said photocathode and means for accelerating the photoelectrons emitted by said photocathode into incidence with said target member to generate a charge image on the opposite side of said target with respect to said photocathode corresponding to the electron excitation, said target positioned in proximity to said photocathode to provide focusing of the photoelectrons emitted by said photocathode between said photocathode and said target, said faceplate and said photocathode transmissive to a second portion of said input radiation including the infrared region directed onto said pickup tube and focused onto said target member, said target member exhibiting a response to said second portion of said input radiation directed thereon by generating a charge image on the opposite surface of said target with respect to said photocathode corresponding to the radiation within said second portion, and means for generating and directing an electron beam over the opposite side of said target with respect to said photocathode to derive an electrical signal representative of the resultant charge image.
 2. The device set forth in claim 1 in which said target member comprises a diode.
 3. The device set forth in claim 1 in which said target comprises a semiconductive wafer including a plurality of P-N junctions localized near the surface of said target and remote from said photocathode, means for reverse biasing said P-N junctions comprising said means for generating said electron beam and means for deriving an output current from the reverse bias charging of the P-N junctions by said electron beam.
 4. The device set forth in claim 1 in which switching means is provided for modifying said means for accelerating said photoelectrons to thereby modify the response of said tube to said first portion of said input radiation.
 5. The device set forth in claim 1 in which said target comprises a semiconductive wafer including a plurality of regions of semiconductive material localized near the surface of said target and forming P-N junctions with said wafer, said regions on the surface of said wafer remote from said photocathode, electrical conductive contacts provided on said regions and reflective to radiations in the infrared region. 